1. Field of the Invention
The present invention relates to a scanning probe. More specifically this invention relates to a scanning probe based on the contact system, which is driven along a surface of an object to be measured.
2. Description of Related Art
In a scanning probe used, for instance, in a surface texture measuring instrument for measuring roughness or waviness of a surface of an object to be measured (workpiece), it is important for improving the measuring accuracy to minimize a contacting force (measuring force) between a tip of a measurement stylus (stylus, contact needle) and a surface of a workpiece and maintain the contacting force at a constant level. There has been proposed a scanning probe, a measuring force of which can be minimized and maintained at a constant level (Refer to, for instance, Japanese Patent Laid-Open Publication No. Hei 8-43066).
The scanning probe described in the reference (a probe head of a coordinate measuring machine) includes an electromagnetic measuring force generator for generating an adjustable measuring force in each of the three orthogonal directions (X, Y, and Z directions). In this scanning probe, a measuring force can be minimized and adjusted to a desired level by loading, in response to displacement (speed) of a measurement stylus, a signal reverse to the displacement to a measuring force generator from a control circuit.
With the scanning probe described in the aforesaid reference, however, when a measuring force is minimized by controlling the measuring force generator, also a driving force for a measurement stylus is reduced, and the tracking capability of the measurement stylus to a surface of a workpiece becomes lower. To overcome this phenomenon, it is required to ensure the tracking capability of the measurement stylus by reducing the driving speed for driving the scanning probe, so that the measuring speed cannot be raised with the measuring time delayed.
Further, as a weight of a movable portion operating together with the measurement stylus is large, when a measuring force is reduced, acceleration acting over the measurement stylus when driving the scanning probe may affect the precision in measurement. Namely when the measurement stylus moves with acceleration, the inertial force defined by a product of the acceleration multiplied by a weight of the measurement stylus acts in a direction reverse to the direction in which the measurement stylus moves, and a value including this inertial force is detected as the measuring force. Therefore, the inertial force not causing any severe problem when the measuring force is large may sometimes affect a result of measurement as an error measurement and disadvantageously make it impossible to obtain the desired precision in measurement.